Process for producing metal coated glass-ceramic articles

ABSTRACT

A method of producing a metal coated glass-ceramic article which comprises melting a glass-forming batch containing a nucleating agent and 0.05 to 5 percent by weight, calculated as the metal based on the total weight of the glass-forming batch, of at least one metal compound selected from copper compounds and silver compounds, forming the melt into a glass article of desired configuration, heating the formed glass article in a reducing atmosphere to devitrify the glass, while causing the metallic ions generated from said metal compound by said reducing atmosphere to migrate through the glass matrix and diffuse to the surface of said devitrified article and to reduce the metallic ions to the state of metallic particles on the surface, heating the article in an oxidizing atmosphere to oxidize the metallic particles to the state of metal oxide crystalline particles bonded to each other, and heating the so obtained article again in a reducing atmosphere to reduce said metal oxide to its metallic state on the surface of devitrified article.

United States Patent. [191 Kato et al.

[451 Feb. 5, 1974 PROCESS FOR PRODUCING METAL COATED GLASS-CERAMICARTICLES [75] Inventors: Taketosi Kato; Masakazu Umetsu,

' both of Nagoya; Toshio Kobayashi,

Aichi, all of Japan [73] Assignee: Ishizuka Garasu Kabushiki Kaisha,Aichi-ken, Japan [22] Filed: Feb. 23, 1972 [21] Appl. No.: 228,757

[52] US. Cl 65/32, 65/33, 65/60,

[51] Int. Cl C03b C03b 29/00 [58] Field of Search 65/32, 33, 60, 59

[56] References Cited UNITED STATES PATENTS 3,464,806 9/1969 Seki et al65/32 3,490,887 l/1970 Herczog 65/32 X 3,557,576 1/1971 Baum 65/32 X3,639,113 2/1972 Aslanova et al. 65/33 k 3,231,456 l/l966 McMillan 65/32X 3,704,110 11/1972 Finn 65/32 Primary Examiner-Frank W. Miga Attorney,Agent, or Firm-Leonard W. Sherman et al.

[5 7] ABSTRACT A method of producing a metal coated glass-ceramicarticle which comprises melting a glass-forming batch containing anucleating agent and 0.05 to 5 percent by weight, calculated as themetal based on the total weight of the glass-forming batch, of at leastone metal compound selected from copper compounds and silver compounds,forming the melt into a glass article of desired configuration, heatingthe formed glass article in a reducing atmosphere to devitrify theglass, while causing the metallic ions generated from said metalcompound by said reducing atmosphere to migrate through the glass matrixand diffuse to the surface of said devitrified article and to reduce themetallic ions to the state of metallic particles on the surface, heatingthe article in an oxidizing atmosphere to oxidize the metallic particlesto the state of metal oxide crystalline particles bonded to each other,and heating the so obtained article again in a reducing atmosphere toreduce said metal oxide to its metallic state on the surface ofdevitrified article.

3 Claims, No Drawings PROCESS FOR PRODUCING METAL COATED GLASS-CERAMICARTICLES This invention relates to an improved process for producingglass-ceramic articles, which have been coated with a conductive coatingof either metallic copper or silver, or of both these metals.

Glass-ceramics or devitroceramics, as is well known, are crystallineceramics, which have been made by the crystallization of a glass bodyunder controlled conditions. Glass-ceramics are'made by using theconventional glass making techniques to melt a glass-forming batchcontaining a nucleating agent and/or a crystallization accelerator,molding the resulting melt and thereafter heat treating the shapedarticle under controlled conditions. The shaped glass article isdivitrifed by this heat treatment. In other words, the shaped glassartilce is transformed into a glass-ceramic composed of minute crystalsdispersednearly uniformly throughout the glass matrix. I

One method for forming a metallic coating on the surface ofglass-ceramic articles is disclosed in Japanese Pat. No. 479,655, U. S.Pat. No. 3,464,806, German Pat. No. 1,496,540, French Pat. No. 1,383,611and British Pat. No. 944,571. The first four of the foregoing patentsand the instant invention are assigned to a common assignee. This methodis characterized in that a small quantity of either a copper or silvercompound in addition to the nucleating agent is incorporated into thestarting glass batch used and the devitrification heat treatment iscarried out in a reducing at-- mosphere. During the process ofcrystallization of the glass by the devitrification heat treatment themetal compound, which has been incorporated in the glass batch, migratesthrough the glass matrix to the surface of the article in the form ofmetallic ions. The metallic ions, which have become diffused atthesurface, are reduced to minute metallic particles on exposure to thesurrounding reducing atmosphere, with the consequence that aglass-ceramic article having a metallic coating layer or film isobtained. However, a major portion of the fine-grained metallicparticles formed by reduction in such a method are. present as indepentparticles; Therefore, a continuous metallic film layer is not produced.This coating consisting of minute metallic particles can be changed to acontinuous metallic layer by submitting the surface of the article to amechanical step such as a buffing treatment, thus flattening and bondingthe metallic particles by the rubbing action to obtain an even andsmooth continuous surface. However, great diffculty is experienced inbuffing of the whole surface in the case of inner surfaces of finetubular articles or articles having a complicated configuration. i

We have now found that in making metalcoated glass-ceramic articles asmooth, continuous phase metallic coating can be directly formed when,following the hereinbefore described heat treatment in a reducingatmosphere, the article is submitted to a heat treatment in an oxidizingatmosphere and thereafter again submitted to a heat treatment in areducing atmooxide by the second heat treatment in an oxidizingatmosphere, with the consequence that the crystalline form becomesenlarged and particles adjacent to each other becomes bonded to eachother, after which the metal oxide crystals, while still retaining theirbonded state, are again reduced to their metallic form by the third heattreatment in a reducing atmosphere. This resulting continuous metalliccoating can be observed by an electron microscope, and also can be shownby a measurement of its conductivity.

Thus, a continuous metallic coating whose particles are in intimatecontact with each other is formed according to the instant invention thebasis of the foregoing discovery. Therefore, there is no necessity for abuffing treatment of the article. Further, since the in-. vention isapplicable to glass-ceramic articles of any configuration, thedifficulty experienced in the prior art in buffing articles ofcomplicated configuration is obviated. Again, the metallic coatingformed by this method, being integrally adhered to the glass-ceramicbody, has a stripping force far greater than the metallic coatingsobtained by such other methods as vacuum evaporation, painting, baking,spattering or the like, its value being about 1.0-2.0 kg/mm and anaverage of about 1.5 kglmm Further, it becomes possible according theinvention method to control thethickness and structure of the metalliccoating of the final product by adjusting the amount added to thestarting glass batch of the metal compound for the coating and/or theconditions under which the heat treatment is carried out in theoxidizing atmosphere. Therefore, a metal coated glass-ceramic articlehaving the desired conductivity can be otained. Thus, the present methodis very effective for making metal coated glass-ceramic articles havingthe required conductivity, which make them useful as the base materialof printed circuits, capacitors, electronic components and the like. Inaddition, the products of the invention are suitable for making variousarticles for daily use, such as decorative items and the like, sincethey have a fine metallic 'luster. If necessary, the formed coatedsurface can be further plated with such metals as copper, silver andother metals which can be applied by the electroplating technique.

It is therefore an object of this invention to provide an improvedprocess for producing glass-ceramic arti- Cles which have a closelybonded metal coating of either copper or silver, or of both thesemetals. Another object is to provide a process for producing a metalcoated glass-ceramic article having a controlled conductivity.

Other objects and advantages of the invention will become apparent fromthe following description.

According to the present invention there is provided a process forproducing a metal coated glass-ceramic article which comprises melting aglass-forming batch containing a nucleating agent and 0.05 to 5 percent,calculated as the metal based on the total weight of the glass-formingbatch, of at least one compound of a metal of the group consisting ofcopper compounds and silver compounds, forming the melt into a glassarticle of the desired configuration, submitting the formed glassarticle to a first heat treatment in a reducingatmosphere to devitrifythe glass, while causing the metallic ions generated from said metalcompound to migrate through the glass matrix and diffuse to the surfaceof said devitrified article and to reduce the metallic ions to the stateof metallic particles on the surface, followed by a second heating ofthe article in an oxidizing temperature to oxidize the metallicparticles to the state of metal oxide crystalline particles bonded toeach other, and thereafter a third heating of the resultant articleagain in a reducing atmosphere to reduce said metal oxide to itsmetallic state, whereby the metal coating is formed on the surface ofthe devitrified article.

As previously indicated, the method of producing a metal coatedglass-ceramic article by melting a glassforming batch containing anucleating agent and a metal compound, the source of the metal coating,then forming the melt into the desired shape, and thereafter heattreating the resultant article in a reducing atmosphere is known.Therefore, the most important feature of the present invention residesin submitting the article, following its heat treatment in the aforesaidreducing atmosphere, to a second heat treatment in an ozidizingatmosphere and thereafter finally to a third heat treatment again in areducing atmosphere.

The preferred embodiments of the invention will be more fully describedbelow.

Almost all known glasses are metastable at room temperature with respectto the more stable crystalline state, and thousands of glasscompositions, comprising all of the commonly usedglass makingingredients, have already been sucessfuly crystallized to glassceramics.Therefore, the present invention is not restricted as to the compositionof the glass-forming batch. However, typical glass compositions that arepreferred for use in the instant invention are those of thesilica-alumina-lithia, silica-alumina-lithia-magnesia,silica-aluminalithia-zinc oxide, silica-aluminamagnesia,silica-alumina-calcium oxide and silica-lithia systems. Theamounts ofthe ingredients contained in these compositions may be, on a weightbasis, in the range of 40 88 percent silica, l 35 percent alumina, 0.540.7 percent lithia, 0.5 30 percent magnesia, l 30 percent zinc oxideand 1 15 percent calcium oxide/In addition, other ingredients may alsobe present, if desired. For'example, up to percent of boron oxide, up to22 percent of sodium oxide and/or potassium oxide, and up to percent oflead oxide may be contained in the compositions.

As the nucleating agents and crystallization accelerators, variousagents have also been known. All of these known agents are useable inthe present invention. The preferred nucleating agents, include titania,zirconia,

fluorine, phosphorus pentaoxide, titania-zirconia,titania-zirconia-fluorine, zirconia-fluorine, titaniafluorine,phosphorus pentoxide-fluorine, titaniaphosphorus pentoxide,zirconia-phosphorus pentoxide, titania-phosphorus pentoxide-fluorine,zirconiaphosphorus pentoxide-fluorine, titania-zirconiaphosphoruspentoxide and titania-zirconia phosphorus pentoxide-fluorine. Theseagents are used in amounts, based on the total weight of theglass-forming batch,'of 0.5 percent titania, 0.5 15 percent zirconia,0.2 15 percent fluorine and 0.5 20 percent phosphorus pentoxide. Inaddition to the foregoing nucleating agents, useable also are calciumfluoride, tin oxide, beryllium oxide, chromium oxide, vanadium oxide,nickel oxide, arsenic oxide and molybdenum oxide, each of which may beused in the range of l 10 percent.

The metal compound to be incorporated in the glassforming batch asthesource of the metal to be coated is a compound of either copper orsilver, or a mixture thereof. Suitable compounds, include the oxides ofthese metals or the compounds capable of being converted to the oxidesin an oxidizing atmosphere at elevated temperatures, -i.e., practicallyall of the usually available metal compunds. Typical are the oxides,halides, sulfites, sulfates, nitrates, phosphates and hydroxides ofeither copper or silver. These metal compounds are added in an amount of0.05 5 percent, calculated as metal based on the total weight of theglassbatch, to ensure a sufficiently coherent and stable coating and toprevent interference with the migration of the ions in devitrificationstep.

The glass-forming batch containing the above nucleating agent and metalcompound is first melted in accordance with the conventional glassmaking technique and then formed into a transparent glass article of thedesired configuration. This is followed by submitting the formed glassarticle to devitrification conditions in a reducing atmosphere. Theconditions may be those which have been previously known as conditionsfor the manufacture of glass-ceramics. That is, the article is graduallyheated up to a temperature above the transition point of glass and heldat a temperature at which the crystallization of glasstakes place. Theheating rate should preverably not exceed 5 C. per minute (300 C. perhour). The temperature at which the article is held is a temperaturebetween the transition point of glass -and the melting point of themetal to be coated. The

melting point of copper is 1,080 C. and that of silver is 963 C. Whilethe transition point of glass will vary depending on thebatchcomposition used, it can be confirmed from the measureddifferential thermal curve. The holding time is usually from about 15minutes to about 5 hours.

In forming the reducing atmosphere, hydrogen, carbon monoxide and suchcombustible gases as methane, ethane, propane, butane and town gas canbe used.

In the process of the above described heat treatments, metallic ionscorresponding to the compounds of copper or silver added are formed, andowing to their high mobility they migrate through the glass matrix anddiffuse to the surface of the glass article. The migrationof the ions tothe surface of the article becomes faster as the temperature rises, andtheamount of migration increases with the passageof time. A-gradient inthe concentration of metallic ions inwardly from the surface is seenwhen a devitrified article is observed by means of an election microX-ray analyzer, and the presence of a very striking peak in theconcentration distribution is noted in the region 5 20 microns deep fromthe surface.

The metallic ions which have been thus diffused to the surface of thearticle during the devitrification process come into contact with thesurrounding reducing atmosphere and are reduced to their metallic form.A major portion of the reduced metal becomes discretely deposited on thewhole of the article surface as minute crystalline particles. It will beunderstood that the thickness of the deposited metal layer may be.controlled by varying the amount of metal compound used and/or thetreating temperature and time.

Following the aforesaid heat treatment in a reducing atmosphere, asecond heat treatment in an oxidizing temperature is carried out. Theoxidizing atmosphere can be formed of either oxygen or anoxygencontaining gas such as air. Prior to the introduction of theoxidizing gas, the previously present reducing gas is convenientlypurged with an inert gas such as nitrogen or helium. The temperaturethat is preferably employed in this heat treatment step is one which isin the range between 200 C. and the melting point of the coated metal.The minute-crystalline particles of metal deposited on the surface ofthe devitrified article in the prior step is oxidized by exposure to theoxidizing atmosphere at an elevated temperature in this second heattreatment step and is converted to a metal oxide. Since the discrete,minute metallic particles change their form and become larger in sizeduring their transformation to an oxide, the oxidized particles becomebonded to each other. The degree of the bonding depends on thetemperature and time of the heat treatment and is enhanced concomitantlywith an increase in the temperature and/or increase in the time. Inaccordance with the degree of bonding that is desired, the heattreatment time can be varied from a short period of time, say, about onesecond to about 1.5 hours within the above noted temperature range.

The product resulting from the foregoing second heat treatment isfinally submitted to a third heat treatment in a reducing atmosphereagain. This reducing atmosphere is the same as that described inconnection with the first step, while the heat treatment temperature isi one between 200 C and the melting point of the coated metal, as in thecase with the second step. The metal oxide coating formed on the articlesurface during the second step is again reduced to its metallic form byexposure to the reducing atmosphere in this third step. In this case,the metal oxide particles, which have been bonded in the second step,are reduced while maintaining their bonded state intact. Hence, thereduced metallic particles are also bonded to each other to form acontinuous phase. This clearly differs from the noncontinuous phaseobtained in the first step, a phase composed of reduced particlesdiscretely deposited on the surface of the article. While the heattreatment time in the third step must be one in which at least a part ofthe metal oxide is reduced to metal and will vary depending on thetemperature employed, it is usually a minimum of about 30 seconds. It isnot necessarily required that all of the metal oxide particles arecompletely reduced. The degree of reduction can be suitably chosen inaccordance with the conductivity desired in the final product.Thereafter, the article is allowed to cool to room temperature underconditions which do not oxidize the metal coating formed by reduction.Thus, the final product is obtained. I

As can be appreciated from the foregoing discussion, the magnitude ofthe conductivity of the final product not only depends upon the amountof the metal compound incorporated in the starting glass-forming batchbut also can be controlled principally by the degree of bonding of themetal oxide particles during the second heat treatment step as well asthe conditions of the third heat treatment step. The temperatures andtimes of the heat treatments that are employed for obtaining the desiredconductivity values can be readily determined by means of routineexperimental work.

The final product obtained by the above described method has on itssurface a strongly coherent coating consisting of either copper orsilver, or of both these metals, and having the desired conductivityvalue. And the crystalline structure of the glass-ceramic body ischiefly dependent on the composition of the starting glass batch and isconfirmed by X-ray diffraction analysis to be either B-euqriptite,fi-spodumen, anorthoclase,

mitted to the first, second and third heat treatments, in accordancewith the method of the present invention.

The first heat treatment was carried out in an atmosphere of hydrogen,the second heat treatment, in an atmosphere of air, and finally thethird heat treatment was again carried out in an atmosphere of hydrogen.in all cases, a glass-ceramic article coated with copper was obtained.

Runs 1 8 were carried out, the results of which are shown in Table l.While the copper ingredient for forming the metallic coating is shown asCuO in the table, in Run 7 copper sulfate is used.

The heating rate shown in the table is the rate at which the temperaturewas raised per hour to heat the sample up to the maximum temperature inthe first heat treatment. And the electrical resistance (0.) is theresistance of the final product, as measured between the two ends of thecylindrical sample.

TABLE I Run No. l

TiO P 0 BeO Cu O Heating rate (C/hr) First heat treatment Maximum temp.(C) Holding time (hr.)

TABLE I Continued Run No. l 2 3 4 5 (1 7 8 Second heat treatmentTemperature (C) 400 800 800 350 600 550 700 900 Holding time (min.) 20L2 L6 3 5 [O l V; Third heat treatment Temperature (C) 400 800 800 300600 400 700 800 Holding time (min.) 30 I l5 3 4 A l Resistance (9) l lI5 500 1 1 1 10 "In the first heat treatment in the case of Runs 4 andshown in the table.

EXAMPLE 2 A glass of the following composition (wt. percent) was used,and cylindrical samples identical to that described in Example I wereprepared.

SiO 53.4 AI O 6- LiO L9 M go 7.6 Cato 5.4 13.0,, 0.8 Na O 4.0 ZrO 2.7 F6.5 Cu O l.()

In the first heat treatment these samples were raised to a temperatureof 950 C. by heating in every instance at the rate of 170 C. per hour,which temperature was held for one hour. This was followed by submittingthe samples to the second and third treatments while varying theconditions. Fourteen runs (Nos. 9 22) were carried out in this manner.The surrounding atmospheres employed in the several steps were the sameas those of Example 1. The results obtained are shown in Run No.

Glass composition (wt.

, 2 2 3 Li,0 MgO C220 2 Na O K 0 PbO M00 CaF ZrO F TiO LIIW Ch) I l 5.the samples were held for a half hour at 6003 C. priorto the half hourat 820 C.

Table II. It can be appreciated that the electrical resistance of thecoating of the final product can be controlled by varying the conditionsunder which the second and third heat treatments are given.

Table II Run Second heat Third heat trcatmenElectrical resistance No.treatment 1 Temp. (C) Time Temp. C) Time (0) (min.) (min.)

9 950 V4 950 A 2 I0 950 A 950 20 500 l l 850 850 I 80 I2 850 I5 850 40 3I3 750 l/l2 750 20 900 I4 750 3 750 30 1 15 650 a 650 1 I00 16 650 5 650l0 1 17 550 A 550 1 1 I8 550 5 550 20 1 19 450 M 450 30 1 20 450 5 45020 30 2| 350 l 350 -30 I 22 350 40 350 l EXAMPLE 3 Example 1 wasrepeated except that silver compounds were used as the compound forforming the metallic coating. Seven runs (Nos. 23-29) were conducted. Asthe silver compound, silver chloride was used in Runs 25 and 28, whilesilver nitrate was used in the rest of the runs. In every instace aglass-ceramic body having a strong closely bonded silver coating wasobtained. The compositions of the glasses, the heat treatment conditionsin the several stages, and the electrical resistances of the resultingproducts are shown in Table III. The terms used in the table have thesame meanings as those of Table l.

I l El 51 l l 3| 1 I l l l l TABLE III Continued Run No. 23 24 25 26 '2728 29 BeO I.0 Ag,0 L0 L0 L0 1.0 0.5 L0 L0 Heating rate (C/hr.) I70 I70I70 I70 I70 I 170 I70 First heat treatment Maximum temp. (C) 850 800*800* 850 850 900 900 Holding time (hr.) I A k I 2 I 1 Second heattreatment Temperature (C) 350 750 550 650 800 700 900 Holding time(min.) 2 5 Z 1% I A Third heat treatment Temperature (C) 350 750 550 650800 700 700 Holding time (min.) I0 I 3 I I Q 3 Resistance ((1) 200 I l30 30 70 In the first heat treatment in the case of Runs 24 and 25, thesamples were held for I hour at 600C. prior to the one hour at 800C.indicated in the table.

' EXAMPLE 4 Example I was repeated but using as the metalliccoat-forming compound both a copper compound and a silver compound inconjunction. Six runs (Nos. 30-35) were carried out. In each run thetemperature was raised to the first heat treatment temperature at therate of I70 C. per hour, and in every instance a glassceramic bodyhaving a strongly adherent metallic coating consisting of both copperand silver was finally obtained. The compositions of the glass batches,the heat treatment conditions of the several stages, and the electricalresistances of the resulting products are shown in Table IV. The termsused in the table have the same meanings as thoseof Table I. In thecaseof the first heat treatment of Run 31, the sample was held for one hourat 600 C. prior to the half hour at 800 C. shown in the table.

W hat is claimed is:

1. In the method of producing a metal coated glassceramic article, thecrystalline structure of the glassceramic being B-eugriptite,B-spodumen, anorthoclase, diopside or anthophirite, by melting aglass-forming batch containing silica and alumina as the main componentsand a nucleating agent and 0.05 to 5 percent by weight, calculated asmetal basedon the total weight of the glass-forming batch, of at leastone metal compound selected from copper compounds and silver compounds,forming the melt into a glass article of desired configuration, andheating the formed glass article in a reducing atmosphere to devitrifythe glass, while causing the metallic ions generated from said metalcompound by said reducing atmosphere to migrate through the glass matrixand diffuse to the surface of said devitrified article and to reduce themetallic ions to the state of metallic particles'on the surface, the

Run No.

Glass composition (wt.

2 A1 0 LL 0 MgO CaO B 0 Na O K 0 PbO M00 CaF ZrO F TiO P 0 BeO GU 0 A oFirst heat treatment Maximum temp. (C) Holding time (hr.)

Second heat treatment Temperature (C) Holding time (min.)

Third heat treatment Temperature (C) Holding time (min.)

Resistance (0.)

improvement which comprises carrying out the heat treatment inthreestages: a first step of heating the formed glass article in a reducingatmosphere to a temperature between the transition point and the meltingpoint of the metal with which the article is to be coated, at a rate notexceeding 300 C. per hour and maintaining said temperature for a periodof time of from about 15 minutes to about 5 hours, followed by a secondstep of heating the article in an oxidixing atmosphere, at a temperaturebetween about 200 C. and the melting point of said metal for a period oftime of from about one second to about 1.5 hours, to oxidize by saidoxidizing atmosphere at least a part of the metallic particles to thestate of metal oxide crystalline particles bonded to each other, andthereafter a third step of heating the so obtained article again in areducing atmosphere, at a temperature between about 200 C. and themelting point of said metal for a period of time of at least about 30seconds, to reduce by said reducing atmosphere at least a part of saidmetal oxide to its metallic state on the surface of the devitrifiedarticle.

2. The method of claim 1 wherein the first heat treatment consists inraising the temperature of said formed glass article at a rate notexceeding 170 C. per hour to a temperature between the transistion pointof glass and the melting point of the metal with which the article is tobe coated followed by holding the article at this temperature for aperiod of time from about 15 minutes to about 5 hours.

3. The method of claim 1 wherein the glass-forming batch consistsessentially of:

- a. a glass selected from the group consisting of:

SiO 40-88 percent, A1 10-35 percent and U 0 -30 percent;

SiO 40-88 percent, Al O 1.0-35 percent, Li O 0.5-'-30 percent and MgO05-30 percent;

SiO 40-88 percent, A1 0 -35 percent, Li O 05-30 percent, MgO 10-30percent and Ca0 l-l5 percent; and

SiO 40-88 percent, A1 0 l.035 percent. MgO

10-30 percent and CaO l'-l5 percent b. an added glass compound selectedfrom the group consisting of: B 0 0-10 percent; 0-22 percent of Na O. K0 or mixtures: and PbO 0-l5 percent c. a nucleating agent selected fromthe group consisting of the following compounds and combinations Ti O0.5-20 percent, ZrO 05-15 percent and P 0 05-20 percent; TiO 0.520percent, ZrO 05-15 percent, P 0

05-20 percent and F 02-15 percent; and a combination of one of abovenucleating agents with at leastone member selected from CaF 5,0 BeO, CrO V 0 NiO, AS 0 and M00; in an amount of 1'-l0 percent; and d. at leastone metal compound selected from oxides, chlorides, sulfates andnitrates of copper and silver in an amount of 005-5 percent, calculatedas the metal.

2. The method of claim 1 wherein the first heat treatment consists inraising the temperature of said formed glass article at a rate notexceeding 170* C. per hour to a temperature between the transistionpoint of glass and the melting point of the metal with which the articleis to be coated followed by holding the article at this temperature fora period of time from about 15 minutes to about 5 hours.
 3. The methodof claim 1 wherein the glass-forming batch consists essentially of: a. aglass selected from the group consisting of: SiO2 40-88 percent, Al2O31.0-35 percent and Li2O 0.5-30 percent; SiO2 40-88 percent, Al2O3 1.0-35percent, Li2O 0.5-30 percent and MgO 0.5-30 percent; SiO2 40-88 percent,Al2O3 1.0-35 percent, Li2O 0.5-30 percent, MgO 1.0-30 percent and Ca01-15 percent; and SiO2 40-88 percent, Al2O3 1.0-35 percent, MgO 1.0-30percent and CaO 1-15 percent b. an added glass compound selected fromthe group consisting of: B2O3 0-10 percent; 0-22 percent of Na2O, K2O ormixtures; and PbO O-15 percent c. a nucleating agent selected from thegroup consisting of the following compounds and combinations ofcompounds: TiO2 0.5-20 percent; ZrO2 0.5-15 percent; F 0.2-15 percent;P2O5 0.5-20 percent; TiO2 0.5-20 percent and ZrO2 0.5-15 percent; TiO20.5-20 percent, ZrO2 0.5-15 percent and F 0.2-15 percent; ZrO2 0.5-15percent and F 0.2-15 percent; TiO2 0.5 -20 percent and F 0.2-15 percent;P2O5 0.5-20 percent and F 0.2-15 percent; TiO2 0.5-20 percent and P2O50.5-20 percent; ZrO2 0.5-15 percent and P2O5 0.5-20 percent; TiO2 0.5-20percent, P2O5 0.5-20 percent and F 0.2-15 percent; ZrO2 0.5-15 percent,P2O5 0.5-20 percent and F 0.2-15 percent; TiO2 0.5-20 percent, ZrO20.5-15 percent and P2O5 0.5-20 percent; TiO2 0.5-20 percent, ZrO2 0.5-15percent, P2O5 0.5-20 percent and F 0.2-15 percent; and a combination ofone of above nucleating agents with at least one member selected fromCaF2, SnO2, BeO, Cr2O3, V2O5, NiO, AS2O3 and MoO3 in an amount of 1-10percent; and d. at least one metal compound selected from oxides,chlorides, sulfates and nitrates of copper and silver in an amount of0.05-5 percent, calculated as the metal.